Yes the plugs are an expensive item, you are typically looking at about $100 for a set, why were they replaced, did you request it, modern plugs will usually see 80-100k before they need to be replaced.
Also, assuming you still have your stock intercooler on, you need to factor the additional labour in for removing and replacing in order to gain access to the plugs.
Oil isn't typically included in your base service price either, as different types if cars require different grades and quantities of oil.
As mentioned give them a call on Monday and ask for a more detailed cost breakdown.
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